This invention relates to an ink jet printing process using an ink jet ink composition for improving the ozone and light stability of an ink jet image.
Ink jet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired image. In one process, known as continuous ink jet, a continuous stream of droplets is charged and deflected in an imagewise manner onto the surface of the image-recording element, while unimaged droplets are caught and returned to an ink sump. In another process, known as drop-on-demand ink jet, individual ink droplets are projected as needed onto the image-recording element to form the desired image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging.
The inks used in the various ink jet printers can be classified as either dye-based or pigment-based. A dye is a colorant that is molecularly dispersed or solvated by a carrier medium. The carrier medium can be a liquid or a solid at room temperature. A commonly used carrier medium is water or a mixture of water and organic co-solvents. Each individual dye molecule is surrounded by molecules of the carrier medium. In dye-based inks, no particles are observable under the microscope. Although there have been many recent advances in the art of dye-based ink jet inks, such inks still suffer from deficiencies such as low optical densities on plain paper and poor light-fastness. When water is used as the carrier medium, such inks also generally suffer from poor water-fastness.
A pigment is a colorant that is insoluble in the carrier medium, but is dispersed or suspended in the form of small particles, often stabilized against flocculation and settling by the use of dispersing agents. Many such compounds are known and are commercially used. Color Index International (publication by the Society of Dyers and Colorists, 1997) lists various classes of pigments. It is common practice to produce pigment compositions in the form of finely divided dispersions, which may be generated by well-known methods such as ball milling. In order to obtain the optimum dispersion properties it is common to have present at least one dispersant, and the choice of dispersant is important for achieving acceptable pigment dispersion properties. The purpose of the dispersant is to stabilize the particles and to prevent growth by aggregation and flocculation. However, merely adsorbing a dispersant to the pigment surface may lead to competition for such dispersant from solvents and humectants used in the ink formulation and may lead to desorption. In general, such systems may also suffer from a dependence on the concentration of the pigment, the type of humectants used, and the temperature and pH of the formulation containing the pigment. Therefore, it is often difficult to identify an acceptable dispersant which provides the needed ink stability and is compatible with other components in the ink formulation.
Images obtained from pigment-based inks generally have better light-fastness and ozone-fastness than that of the images obtained from dye-based inks. It is especially true when these are used with a recording element containing a porous image-receiving layer. However, pigment based inks have not received a wide degree of acceptance in ink jet ink systems, because of problems associated with the preparation, performance and reliability of the composition, such as dispersability, print properties, dispersion stability, latency, smear, and gloss.
When a pigment-based ink is formulated, a dispersant is normally used along with a milling or physical grinding step. Alternatively, after milling, the pigment surface may be chemically modified to render the particles dispersible in an aqueous formulation. However, there is a problem with these techniques in that they take time and are expensive. It would be desirable to find alternative colorants having the image permanence of pigments but which do not require milling.
U.S. Pat. No. 5,922,118, EPA 0904327, and WO9747699 disclose surface-modified pigments, wherein such surface modification comprises ionic or ionizable groups for improvement of pigment dispersability. However, these pigments still require a milling step.
EP 1146094 describes pigment compositions for paints and inks consisting of mixtures of salts of quinacridone monosulfonic acids and quinacridone pigments. The quinacridone monosulfonic acid derivatives of this reference are not water-soluble and the pigment compositions require mechanical milling to achieve acceptable dispersions.
U.S. Pat. Nos. 6,066,203 and 5,368,641 describe mono- and bis-sulfamoyl (xe2x80x94SO2NRRxe2x80x2) derivatives (respectively) of quinacridones for use in formulating quinacridone pigment dispersions similar to those described in EP1146094 above.
It is an object of the invention to provide an ink jet printing process using an ink jet ink composition that employs self-dispersed particles that do not require milling or grinding and do not require the use of a dispersant.
This and other objects are achieved in accordance with the present invention which comprises an ink jet printing process comprising the steps of:
A) providing an ink jet printer that is responsive to digital data signals;
B) loading the printer with ink jet recording elements comprising a support having thereon an image-receiving layer;
C) loading the printer with an ink jet ink composition comprising water, humectant and a self-assembling colorant that is capable of spontaneously forming a nanoparticulate dispersion without any prior physical attrition or surface modification, the colorant having the formula:
(A)m-Q-(Z)n
xe2x80x83wherein: Q represents a chromophore;
each A independently represents an organic or inorganic group capable of hydrogen bonding or other non-covalent bonding;
each Z independently represents an organic or inorganic group capable of electrostatic bonding; and
m and n each independently represents an integer from 0 to 10;
xe2x80x83with the proviso that n+m is at least 1; and with the further proviso that at least about 50 wt. % of the colorant is present in the composition as particles; and
D) printing on the image-receiving layer using the ink jet ink in response to the digital data signals.
It was found that the ozone and light stability of an ink jet image was improved when printing using the composition described herein.